package LC;

import java.util.ArrayList;
import java.util.List;

/**
 * https://leetcode.com/problems/combination-sum-iii/description/
 * Find all possible combinations of k numbers that add up to a number n,
 * given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
 * Example 1:
 * Input: k = 3, n = 7
 * Output:
 * [[1,2,4]]
 * Example 2:
 * Input: k = 3, n = 9
 * Output:
 * [[1,2,6], [1,3,5], [2,3,4]]
 */
public class LC_216_CombinationSumIII_DFS {
    public static void main(String[] args) {
        System.out.println(Solution.combinationSum3(3, 9));
    }

    static class Solution {
        static List<List<Integer>> combinationSum3(int k, int n) {
            List<List<Integer>> res = new ArrayList<>();
            if (k < 1 || n < 1) return res;
            List<Integer> cur = new ArrayList<>();
            dfs(res, cur, 0, k, n, 1);
            return res;
        }

        static void dfs(List<List<Integer>> res, List<Integer> cur, int sum, int k, int n, int level) {
            if (sum == n && k == 0) {
                res.add(new ArrayList<>(cur));
                return;
            } else if (sum > n || k <= 0) return;
            for (int i = level; i <= 9; i++) {
                cur.add(i);
                dfs(res, cur, sum + i, k - 1, n, i + 1);
                cur.remove(cur.size() - 1);
            }
        }
    }
}